1. Field of the invention
The present invention relates to a fluid bearing unit in which the dynamic pressure of fluid is utilized.
2. Related art
Conventionally, various drive units for driving a rotational body are well known, in which the shaft and the bearing are supported through a fluid bearing unit so that they can be relatively rotated.
For example, in a spindle motor for driving a disk, a lubricant such as oil (bearing fluid) is charged between a radial sliding bearing made of stainless steel stipulated as SUS or copper alloy and a shaft made of stainless steel stipulated as SUS, and a non-contact sliding section is formed by the dynamic pressure of bearing fluid, so that the shaft and the bearing can be relatively rotatably supported.
However, in the fluid bearing unit in which the shaft is made of stainless steel stipulated as SUS and the radial bearing is made of stainless steel stipulated as SUS or copper alloy, the viscosity (coefficient of viscosity) of bearing fluid is changed in accordance with a change in the temperature of an environment in which the rotational body drive unit is used. Therefore, the fluid bearing unit is greatly affected by the change in viscosity.
For example, when a diameter of the shaft is 4 mm and a clearance of the bearing is 4 mm and a change in the environmental temperature is .+-.30.degree. C. with respect to the normal temperature, and when the bearing fluid is a common oil, the viscosity is reduced to 1/3 of the normal value on a high temperature side, and the viscosity is increased to 3 times of the normal value on a low temperature side.
In this connection, the dynamic pressure of bearing fluid is in inverse proportion to the square of a clearance formed between the shaft and the radial sliding bearing. Also, the dynamic pressure of bearing fluid is in proportion to the viscosity of bearing fluid.
Accordingly, on the assumption that the dynamic pressure of bearing fluid is affected only by the viscosity, the dynamic pressure is reduced to 1/3 at high temperature.
Also, the bearing loss (viscosity resistance) is in inverse proportion to the clearance and proportion to the viscosity. Accordingly, on the assumption that the dynamic pressure of bearing fluid is affected only by the viscosity, the bearing loss is increased to 3 times at low temperature.
In this connection, Japanese Unexamined Patent Publication No. 5-118322 discloses a technique characterized in that the shaft is made of material, the thermal expansion coefficient of which is higher than that of material composing the radial sliding bearing. According to this technique, a decrease in the dynamic pressure at high temperature and an increase in the bearing loss at low temperature can be prevented.
However, in the fluid bearing unit described above, the radial sliding bearing is made of ceramics. When ceramics is used for the bearing, machining is complicated, so that the cost is raised. Further, it is difficult to enhance the dimensional accuracy.